A technique for driving a switching element of an inverter by comparing a command voltage for a rotary machine with a carrier wave is commonly used. The technique allows a pseudo sinusoidal command voltage to be applied to a terminal of the rotary machine.
The technique however may cause an output voltage of the inverter to binarily fluctuate. This may highly change a voltage at a neutral point of the rotary machine, resulting in that common-mode noise may occur and/or surge voltages may increase.
In order to address such disadvantages, a control system is disclosed in U.S. Pat. No. 7,274,173 corresponding to Japanese Patent Application Publication No. 2006-136125.
The control system disclosed in the US patent provides a control system for a three-phase rotary machine. The control system is designed to convert a voltage across a capacitor of a DC (Direct Current) to DC converter into a desired voltage relation to a power supply voltage to the DC to DC converter.
The control system is also designed such that a terminal of each phase of the three-phase rotary machine is electrically connected across the capacitor of the DC to DC converter. With the configuration of the control system disclosed in the US patent, a substantially sinusoidal voltage is applied to each phase of the three-phase rotary machine, thus reducing surge voltages.